COSC 3P92
Cosc 3P92
Week 11 Lecture slides
Violence is the last refuge of the incompetent.
Isaac Asimov, Salvor Hardin in "Foundation"
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COSC 3P92
Input/ Output
• There are two general types of I/O devices:
1. physical I/O devices
eg. output port to printer
2. virtual (logical) I/O devices: operating system abstractions eg.
spooled print file, files vs. sectors on disk
» Telephony Application Programmed Interface (TAPI)
» Telephony Service Provider Programming Interface (TSPI)
Physical I/O
• To transfer information between CPU and physical
I/O devices, one may use the following
techniques:
1. programmed I/O
2. interrupt (-driven) I/O
3. direct memory access (DMA)
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COSC 3P92
1. Programmed I/O
• I/O ports may be addressed using:
1. standard (isolated) I/O address space
– eg. Intel
– In and OUT instructions
– pin on CPU chip indicates whether IO or memory address
space being used. [See next slide]
2. memory-mapped I/O address space
– eg. Motorola
– devices reside at specific memory locations.
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COSC 3P92
8088 i/o
port control
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COSC 3P92
1. Programmed I/O
• Advantages Memory-mapped I/O:
–
–
–
–
no special I/O opcodes
all instns that reference memory can access IO
# I/O ports is unlimited
hardware bus simplified
• Disadvantages:
– I/O interfaces may need more circuitry to recognize larger
addresses.
– possible slow down in bus performance.
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COSC 3P92
• An I/O operation may be performed:
1. unconditionally – CPU sends data to device at any time
2. conditionally – checking the status of the device before the
operation (i.e. handshaking).
– CPU may have to poll and wait for device -> inefficient
• The CPU communicates with an I/O devices via
one or more registers called I/O ports.
– Bit-serial ports
» Every bit in the port may be configured as either input or
output.
Data-direction register
0
1
1
0
0
1
1
0
1=output 0=input
I/O port
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COSC 3P92
– Parallel ports
» Each I/O port (as a whole) may be configured as either
input or output.
Command register
0
1
1
0
0
Other control signals
1
1
B
0
A
1=output 0=input
I/O port A (output)
I/O port B (input)
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COSC 3P92
Interrupt I/O
• When an I/O device is ready to send (receive) data
to (from) the CPU, it signals (or interrupts) the
CPU for its attention.
– No need to poll device status.
– As soon as the CPU finishes the current instruction, it
transfers its execution to an interrupt-service routine which
responds to the external interrupt.
Current execution path
Interrupted
Interrupt occurs
Interrupt-service
routine
perform I/O
transfer
Resume
Return from
interrupt
• Q. How does the CPU know which one of the
ISRs to execute when there is more than one?
– Interrupt Service Vectors: address of interrupt service routines,
commonly kept in special jump table.
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COSC 3P92
Interrupt I/O
example code:
Setting up interrupt I/O...
move.b
move.b
move.b
move.b
#$81,
#$00,
#$81,
#$01,
DDRA
DDRB
PORTA
PORTA
; data direction
;
registers
; start pulse
; to device
...
Device will cause interrupt when ready/done,
and an interrupt routine will complete
transaction...
move.b
PORTB, D1
; interrupt service rtn.
rte
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COSC 3P92
Interrupt I/O (continued)
•
Polled
•
Daisy-chain
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COSC 3P92
Direct Memory Access (DMA)
• It is a technique of transferring data between
memory and I/O devices without CPU
intervention.
– CPU sets up transfer with DMA controller; then transaction
occurs without CPU
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COSC 3P92
Direct Memory Access (DMA)
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COSC 3P92
DMA
3 techniques:
1. Block transfer
- whole data block transferred
- CPU can do non-related bus activities in the
meanwhile
2. Cycle stealing
- DMA controller freezes the CPU, and then
does a DMA while CPU frozen
- word-by-word transfer
3. Interleaved
- DMA controller uses CPU cycles that aren't
using the bus, letting DMA xfers and CPU
alternate use of the bus
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COSC 3P92
I/O Processor (data channel)
• Independent dedicated I/O processors (smart
DMA controllers) are used in mainframe computer
systems to communicate with I/O devices.
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COSC 3P92
I/O processors
• Each IOP uses DMA to communicate with devices
• Some considerations:
– priority system for IOP's sharing system bus
– priority system for devices on one IO bus
– IOP requires software or hardware to manage different devices
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COSC 3P92
IO example: LED
• Encoder, part of kbd.
– Converts key press into ASCII encoded byte.
– Bus Interface communicates data to I/O bus.
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COSC 3P92
8086 Programmed IO
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COSC 3P92
8086 Programmed IO
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COSC 3P92
68000 IO
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COSC 3P92
68000 IO
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68000 memory
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COSC 3P92
The end
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